High capacity optical transmission links use a Wavelength Division Multiplexed (WDM) physical layer to provide very high bandwidth (BW) on a single fiber. The WDM signals can be carried for very large distances (up to a few thousands of Kilometers) over a single fiber without the need for Optical-Electronic-Optical (OEO) regeneration.
In the past, most such transmission links were configured as simple point-to-point (P2P) links, where all WDM signals are transmitted from one end of the link to the other. Recently, however, optical links have become more complex, with part of the WDM channels being added and dropped at intermediate sites along the link using fixed or reconfigurable Optical Add/Drop Multiplexers (OADMs). The added or dropped channels may undergo electrical conversion at the intermediate sites, or they may be directly connected to another optical link without undergoing OEO regeneration, to form optical cross connects (OXCs).
In both P2P links and in more complex links with add/drop capability, optical amplifiers and other optical modules, such as Dispersion Compensation Modules (DCMs), are placed along the link in order to deliver the signal to the receiving terminal with adequate power and with minimal signal distortion. Optical amplifiers can be of many types, but most often are Erbium Doped Fiber Amplifiers (EDFAs), as described for example in U.S. Pat. Nos. 5,225,922 to Charplyvy et. al, 5,812,710 to Seguya et a., 6,049,413 to Taylor et al, and 6,611,641 to Ghera et al.
One major requirement of optical amplifiers is that they comply with various laser safety standards (such as International Standard, “Safety of Laser Products—Part 1: Equipment Classification, Requirements and User's Guide”, IEC 60825-1 and International Standard, “Safety of Laser Products—Part 2: Safety of Optical Fibre Communication Systems”, IEC 60825-2). These standards dictate that the optical amplifiers automatically shut-down once a disruption occurs in the fiber link (e.g. due to open connector or fiber break), thus avoiding harmful radiation being emitted from the fiber link and potential hazards to technicians and equipment. Besides automatic shut-down, it is also required that optical amplifiers automatically start-up once the disruption in the fiber link has been repaired.
Methods and apparatuses for achieving such automatic shut-down and start-up have been described for example in U.S. Pat. Nos. 5,278,686 and 5,355,250 to Grasso et al, 5,428,471 to McDermott, 6,583,899 to Casanova et al, and 6,423,963 to Wu. All refer to optical amplifiers operating within simple P2P links or closed ring links, and describe two methods for achieving automatic shut-down and start-up: (1) based on loss of input signal to the optical amplifier; (2) based on the existence of a special Optical Supervisory Channel (OSC) signal.
These prior-art methods can be understood with the help of FIG. 1, which shows a simple bi-directional P2P WDM optical transmission link 100. The optical link contains a plurality of transmitters 102 for transmitting data over WDM wavelengths, multiplexers 104 to multiplex the various WDM wavelengths, optical amplifiers 106 to amplify the WDM wavelengths along the link, de-multiplexers 108 to demultiplex the WDM wavelengths, and receivers 110 for receiving the transmitted data. In a bi-directional link such as shown in FIG. 1, equipment for the two transmission directions is usually co-located at the same physical site, Thus, the transmitting terminal of one direction is co-located with the receiving terminal of the other, and vice versa. Additionally, amplifiers for opposite directions are co-located at the same amplifier site 112, as shown in the figure. The link may also include OSC transmitters 114 and receivers 116, placed at the terminals and at the amplifier sites, used for transmitting and receiving an OSC signal which serves to provide management and supervisory instructions to the various sites along the link. Furthermore, the OSC may be modulated using an AC tone, in addition to the data modulation, thus facilitating the detection of its presence or absence along the link.
In a method for automatic shut-down and start-up based on loss of input power, a fiber disruption 118 occurring along the link means that the amplifier immediately following the disruption (in the downstream direction) will not receive any input power. Since optical amplifiers typically incorporate an optical tap and detector at the input to the amplifier, the loss of input power can be easily detected, and the amplifier can automatically shut itself down. This will cause the next amplifier in the link to lose input power and shut down, and so on down the link until the receivers themselves detect the loss of input power. This will then cause the transmitters in the opposite direction (co-located at the same site as the receivers) to shut down, followed by shut-down of all amplifiers in the opposite direction, followed by loss of input to the receivers in the opposite direction, followed by shut-down of transmitters in the original direction, followed by shut-down of all amplifier preceding (upstream of) the original fiber disruption. Thus, this chain of events leads to the shut-down of the entire bi-directional link. Alternatively, since amplifiers in opposite directions may be co-located at the same amplifier site 112, loss of input to one amplifier may cause immediate shut-down of the amplifier in the opposite direction, thus speeding up the whole shut-down process. Automatic start-up of the link occurs when the fiber disruption is repaired and when each amplifier along the link consecutively detects input power and automatically powers up.
In a method for automatic shut-down and start-up based on OSC, a similar process as above occurs, except that instead of detecting the input power within the transmission band, the presence or absence of the OSC (which is typically located outside the transmission band) is detected. This is particularly useful when the optical amplifier includes a distributed Raman pre-amplifier working in a backwards pumping configuration (see e.g. U.S. Pat. No. 6,423,963). In such a case, Amplified Spontaneous Emission (ASE) noise may exist within the signal band even after a fiber disruption occurs, thus preventing the amplifier from identifying the loss of input power. However, the fiber disruption causes the OSC to disappear, and this is easily detected either using the OSC AC tone described above, or by directly monitoring the data modulation of the OSC. Automatic start-up occurs when the fiber disruption has been repaired, and the presence of the OSC is detected.
While automatic start-up and shut-down based on OSC is a powerful method, not all systems support an OSC, and those that do typically use proprietary modulation methods and/or AC tones. This means that an amplifier relying on an OSC for start-up and shut-down must be tightly integrated into the host system, which in turn must support an OSC.
Furthermore, both above mentioned methods were designed to work in simple P2P optical links, but they do not function properly in more complex optical link, in which part of the WDM channels are added and dropped at intermediate sites. This is illustrated in FIG. 2, which shows a portion 200 of such a link. In the example shown in the figure, wavelengths 210 are added to the link at one site 208, and dropped from the link at a next site 216. The Added/Dropped wavelengths may undergo OEO conversion, or may be directly transferred to/from other optical links via OXCs. For the sake of clarity, the figure shows only a section of the link, without terminal equipment (transmitters and receivers), and only one transmission direction.
Consider now a fiber disruption 202 as shown in FIG. 2. Due to this disruption an amplifier 206 will experience a loss of input power, and therefore shut-down. Thus, the first group of wavelengths λ1, . . . , λN 204 will not reach an Add unit 208. However, a second group of wavelengths λN+1, . . . , λM 210 is added to the link at Add unit 208 and continues down the link. Thus, amplifiers 212 and 214 will not experience a loss of input power, and will not shut down (indeed they should not shut down, since they need to keep working in order to carry the remaining traffic). Moreover, since amplifiers 212 and 214 continue working, they generate ASE noise within the entire transmission band, including that part of the band occupied by λ1, . . . , λN, even though these wavelengths themselves are not present. The group of wavelengths λN+1, . . . , λM 210 is dropped at a Drop unit 216, and therefore does not reach an amplifier 218. Thus, amplifier 218 does not receive any wavelength at all, since the original wavelength group λ1, . . . , λN 206 is not present due to the disruption 202. However amplifier 218 does receive ASE noise within the wavelength band occupied by λ1, . . . , λN, generated by amplifiers 212 and 214, and thus may not be able to detect the loss of input power condition. Thus, amplifier 218 will not shut-down as required, disrupting the chain reaction previously described in the context of FIG. 1 for shutting down a fiber link.
Regarding the OSC method of shut-down and start-up, due to disruption 202 the OSC does not reach amplifier 206. However, since the section of the link between Add unit 208 and Drop unit 216 continues to work, the OSC is regenerated in this part of the link and needs to be transmitted along the remaining part (following Drop unit 216) in order to pass information to system management at the end of the link. Thus, the OSC will be detected at amplifier 218, and cannot be used to determine shut-down of this amplifier.
Thus, there is a need for automatic shut-down and start-up methods and systems within or combined with optical amplifiers that function correctly in optical links including OADMs.